The permeability of cerebral blood vessels to proteins was studied in the stroke-prone, spontaneously hypertensive rats (SPRSH) by immunocytochemical method (PAP) designed for demonstration of extravasated serum proteins. As controls served the normotensive Wistar-Kyoto rats and spontaneously hypertensive but stroke-resistant animals (SHRSR). In SPRSH, even in the presymptomatic animals, application of peroxidase-antiperoxidase (PAP) method to demonstrate serum proteins revealed very extensive areas of protein extravasations in the white matter and a striking localization of serum proteins in the cytoplasm of the adjacent neurons. The testing of the blood-brain barrier (BBB) with conventional tracers revealed no or only minimal changes of the BBB. This project is completed.